This invention relates to the demodulation of signals modulated by quadrature phase shift keying (QPSK) modulation methods.
A QPSK signal is the superposition of two signals, a carrier frequency modulated by one set of data and referred to as the in-phase (I) channel signal, and the same carrier frequency in quadrature (phase shifted by 90.degree.), and modulated by a second set of data and referred to as the quadrature (Q) channel signal. The QPSK signal is unbalanced if the two channels do not have equal power. QPSK signals are demodulated by first recovering the carrier frequency and then multiplying the QPSK signal by the carrier frequency which results in suppressing the quadrature phase channel and producing a signal containing only the I channel data which can then be demodulated. The Q channel data is recovered by first phase shifting the carrier so that it is in quadrature relationship to the carrier frequency thereby providing a quadrature carrier. The quadrature carrier is then multiplied by the QPSK signal which results in suppressing the signal containing the I channel data and producing a signal containing only the Q channel data.
This method works if the I and Q channels are in perfect quadrature relationship, that is, if they are exactly 90.degree. (.pi./2 radians) apart. There must also be no phase error in the carriers used to recover the data. To the extent that the signals are not in perfect quadrature relationship or that there is phase error in the carrier used to detect the signals, undesired crosstalk between the I and the Q channels will result.
If the I and Q channels have the same magnitude, or more correctly power content, the effect of crosstalk on each channel will be approximately the same. If the power content of one channel is significantly larger than that of the other channel, the percentage effect of crosstalk of the higher power channel on the lower power channel will be significantly larger than the percentage effect of crosstalk of the lower power channel on the higher power channel. Thus the power supplied to each channel is preferably substantially equal in order to equally minimize errors in both channels.
Equal power distribution results in a waste of power where one channel otherwise requires less power than the other channel, such as when audio and video information are transmitted and the audio channel requires considerably less power than the video channel. Excess power requirements result in extra weight, size and heat dissipation equipment for the receiver. These requirements are especially critical in many applications, such as for receivers in spacecraft.
If the phase error introduced into the system could be controlled so that it was very small, the Q and I channels could use correspondingly different amounts of power and still use the above-described demodulation technique. However, maintaining very small phase errors is difficult and expensive, and becomes more difficult and expensive as the desired transmission fidelity increases. A description of known prior art demodulators follows.
U.S. Pat. No. 4,359,692 to Ryan discloses a shift keyed phase detector. The in-phase and quadrature channel signals are squared, and the squared signals are summed and filtered to provide a first phase error signal. The in-phase and quadrature channel signals are also multiplied with each other and filtered to provide a second phase error signal. The in-phase and quadrature channels are cross multiplied with each of the phase error signals and then combined to provide a data signal in phase with the original data signal. Thus, an error correction signal is used after detection of each channel to reduce oscillator error, as contrasted with crosstalk error.
U.S. Pat. No. 4,085,378 to Ryan et al. discloses a balanced QPSK demodulator which includes a modified Costas loop so configured as to cross couple the in-phase and quadrature phase channels such that minimum crosstalk takes place between the two channels. In effect, the two channels are independently demodulated. The Costas demodulation approach entails use of an error signal derived from the multiplication of the in-phase and quadra-phase channels to indicate the crosstalk between channels. The error signal resulting from the crosstalk is used to drive a reference VCO (voltage controlled oscillator). The demodulator is only suitable for balanced QPSK signals.
U.S. Pat. No. 4,092,606 to Ryan discloses a method of demodulation for a QPSK signal in which a phase lock loop having a variable frequency reference signal responsive to the QPSK signal drives the first and second channels. The demodulated signals are combined to derive a variable amplitude error signal for controlling the coherent reference frequency. Thus, VCO (voltage controlled oscillator) modulation (oscillator dither) is used to allow the signal to be processed in an advantageous manner.